The invention relates to a method of fabricating a quartz capacitance pressure capsule. More particularly, the invention relates to a method of surface preparation, electrode curing and vacuum sealing of the parallel plates of the pressure capsule.
The incorporation of pressure sensors into the electronic control systems of automative vehicle poses severe operational requirements for the pressure sensor. These requirements are further heightened when the pressure sensor is used to measure the manifold pressure (MP) or the absolute manifold pressure (MAP) of the engine.
The pressure sensor must operate in a mobile and severely and hostile enviornment which may include any of the following characteristics: varied temperature extremes, excessive shock and vibration and high levels of radiation causing electromagnetic interferences and corrosive gases and liquids. The pressure proximate the intake manifold is rapidly changeable and susceptable to large variations in magnitude (1-4 atmospheres) which may be caused as the result of explosive backfire or may occur if the vehicle is so equipped during the boost phase of the operation of a supercharger or turbocharger; thus requiring a pressure sensor having a large dynamic range and high sensitivity. Irrespective of the above, if the present invention is utilized in the automobile, the requirements of the industry dictate that it must be (1) inexpensive; (2) repeatable; and (3) capable of being mass produced which implicityly requires the use of novel and inexpensive fabrication techniques as opposed to the ion-milling, vacuum deposition methods such as sputter-etching techniques and brazing fabrication steps mentioned by Polye in U.S. Pat. No. 3,858,097, and by Dias, et al in U.S. Pat. No. 4,064,550.
The present invention is a method of fabricating a dual diaphragm quartz capacitance pressure capsule. The pressure capsule comprises a pair of flat flexible fused quartz plates which are separated by a ring of dielectric material (such as a glass frit) defining an interior chamber which is maintained at a determinable pressure (vacuum) reference level. The pressure capsule contains a plurality of electrodes located within the interior chamber forming the conducting plates of a reference capacitor C.sub.r and a pressure sensing capacitor C.sub.p. In particular, one plate, the upper plate contains a ground electrode while the other plate, i.e., the lower plate contains both the C.sub.p and C.sub.r electrodes. The lower plate may also contain an electrical shield on a surface opposite the internal chamber. In addition, the pressure capsule contains a plurality of electrical contacts, one associated with each of the above electrodes. These contacts are located outside of the internal chamber near the edges of each of the flat plates. The method comprises the steps of (a) preparing the surface of each quartz plate to a determinable flatness; (b) printing the electrodes and electrical contacts thereon using a metal organic ink; (c) curing the electrodes by heating and controlling the temperature profile to remove the organic binders within the ink and to cause the remaining metals to bind or adhere to the quartz plates; (d) printing a slurry glass frit, i.e., the dielectric ring thereon; (e) preglazing the frit to remove non-glass materials in the frit to minimize the formation of a bubble structure developed in later steps; (f) assembling the quartz plates within a registration fixture to form an unsealed pressure capsule or plurality of capsules; and (g) vacuum sealing the unsealed capsule comprising the steps of varying the local temperature and pressure in accordance with a described temperature-pressure profile shown in FIG. 12.
In response to pressure input thereto, both plates act as cantilevered plates, and deflect and bend over the raised dielectric ring varying the capacitance between the plates.
The housing includes means for circumferentially sealing and compressively supporting both plates wherein the compressive forces on the plates are opposingly directed through the dielectric ring. The housing further includes means, such as a port, for communicating the pressure to be sensed to the upper plate proximate the general location of the interior chamber and transfer port means located therein to further communicate the pressure to the second plate.
An advantage of the present invention is that capsule to capsule uniformity is enhanced by minimizing developed bubble structures. In addition, the depositing of electrodes and dielectric material by silk screening permits the achievement of a low cost fabrication process. A further advantage of the process is that pressure capsules having different spacing i.e., capacitance can be achieved. This can be seen from the following: During the vacuum sealing of the capsule, the dielectric is caused to return to a molten, viscous state and by increasing the local pressure, a clamping force is applied to both plates. It can be appreciated that the spacing therebetween will vary with the maximum applied pressure (force) and its time of application.
It is an object of the present invention to fabricate quartz capacitors having a uniform bubble structure in its electrodes and dielectric components.
It is an object of the present invention to seal a pressure capsule in a controlled temperature pressure (vacuum) enviornment.
It is an object of the present invention to provide a pressure capsule having an internal pressure (vacuum) reference and to eliminate the need for vacuum tip-offs.
It is a further object of the present invention to eliminate the need for heavy hold down weights and large thermal masses used during the sealing process.
It is an additional object of the present invention to provide a method of fabrication adaptable to the mass production of quartz pressure capsules.
These and other objects, features and advantages of the invention will be clear from the detailed description of the drawings.